1. Field of the Invention:
This invention relates to a dynamic pressure-type fluid bearing used in a polygon mirror scanner or the like employed in a device such as a laser beam printer.
2. Description of the Prior Art:
In a dynamic pressure-type fluid bearing, a sleeve and a journal are kept out of contact by a fluid film during rotation. Such a bearing is described in the magazine of the Nihon Kikai Gakkai, published in July, 1986, in an article entitled "Newly-developed Air Bearings and their Application to Business Machines" by T. Tanaka. According to this article, it is reported that good results are obtained by applying the fluid bearing to a laser scanner employed in a laser printer.
A basic fluid bearing applied to a laser scanner will now be described with reference to FIGS. 7 and 8. In a dynamic pressure-type fluid bearing using a sleeve and a journal, either the sleeve or the journal may be rotated. In the example of the conventional fluid bearing arrangement described with reference to FIGS. 7 and 8, it will be assumed that the sleeve is rotated.
In FIG. 7, a sleeve 1 rotated at high speed has a radial inner peripheral surface 2. A thrust bearing member 3 having a thrust bottom face 4 is fixedly secured to one end portion of the sleeve 1 and is provided through its center with a passageway 5 for the passage of a gas. The journal 6, which is for axially supporting the sleeve 1, is supported by suitable stationary means 7. The journal 6 includes a radial outer peripheral surface 8 cooperating with the radial inner peripheral surface 2, and a thrust end face 9 cooperating with the thrust bottom face 4. The radial outer peripheral surface 8 is formed to include grooves 10 for producing dynamic pressure.
A magnet 11 is fixedly secured to the outer periphery of the sleeve 1 and opposes plural sets of drive coils 13 and yokes 14 attached to suitable stationary means 12. The lower end of sleeve 1 as seen in the Figure is formed to include an inlet 15 for inflow of a gas, and a pocket (pressure chamber) 16 of very small volume is formed between the thrust bottom face 4 and the thrust end face 9.
In the above arrangement, the thrust bottom face 4 and the thrust end face 9 are in contact when the bearing is at rest, namely when the sleeve 1 is not rotating. When the coils 13 are selectively energized to rotate the sleeve 1, a gas which flows in from the inlet 15 is guided upwardly in the Figure through a small clearance of micron order, which is formed between the radial inner peripheral surface 2 and radial outer peripheral surface 8, in a well-known manner due to the action of the grooves 10. The sleeve 1 is caused to float by several microns due to the pressure of the gas. When the sleeve 1 floats in this fashion, the gas in the pressurized pocket 16 is brought into communication with the outside via the passageway 5. Thus, the gas in pocket 16 is capable of flowing out from the passageway 5, so that the pressure in pocket 16 is self-adjusted in such a manner that balance is maintained among the dead load of the rotational system including sleeve 1, the magnetic attracting force acting upon the rotational system, and the external air pressure acting upon the system. Thus, the sleeve 1 continues rotating at high speed in the levitated state as is well-known in the art.
FIG. 8 illustrates another conventional arrangement, in which portions similar to those shown in FIG. 7 are designated by like reference characters. In this arrangement, as will be apparent from FIG. 8, there are no thrust bearing faces, so that the rotational sleeve 1 is maintained in the levitated state by the magnetic attracting force even when the system is at rest. When the sleeve 1 rotates, a gas flows in from upper and lower inlets 15 as described above due to the action of the grooves 10. Owing to the presence of a thin film of air between the radial inner peripheral surface 2 and radial outer peripheral surface 8 (as a result of which a dynamic pressure-type radial bearing is produced), the sleeve 1 continues rotating at high speed while the contactless state is assured with respect to the journal 6.
In the above-described arrangement of FIG. 7, the gas discharged from the passageway 5 acts so as to maintain the pressure in pocket 16 in a suitable state. Owing to a so-called "orifice effect" produced by the passageway 5, the pressure regulating mechanism is important in terms of regulating a pressure variation within the pocket 16 and has a close relation to a centering effect. Accordingly, the passageway 5 is required to have a very high dimensional precision and positional accuracy. However, machining such a small passageway to satisfy these requirements involves considerable difficulty.
Furthermore, since the periphery of the passageway 5 and the thrust end face 9 repeatedly come into contact whenever the bearing is started and stopped, the rim of the passageway 5 tends to be damaged by friction, as a result of which the reliability and precision of the bearing suffer. If it is attempted to form the thrust bearing member 3 of a wear-resistant material having a high hardness in order to deal with this friction-induced damage, machining of the passageway 5 becomes all the more difficult.
In the conventional arrangement of FIG. 8, on the other hand, the foregoing problems do not arise because there are no thrust bearing surfaces. Also, since the travelling distance of the gas which flows in from the inlets 15 is less than in the arrangement of FIG. 7, the grooves 10 for producing dynamic pressure are easier to design and there is comparatively little risk of a deterioration in characteristics due to frictional loss of the gas.
However, since there is no thrust dynamic pressure bearing effect, the force from magnet 11 for holding the sleeve 1 in the magnetically levitated state becomes unbalanced due to ripple in the current passed through the coils 13. As a result, the sleeve 1 (the rotational system) tends to undergo minute oscillation up and down.